1. Field of the Invention
The present invention relates to an apparatus and methods for high-speed multi-user wireless communication.
2. Description of Related Technology
The idea to use multicarrier methods as a modulation technique is known in the art [Chang, R. W. “Synthesis of band-limited orthogonal signals for multichannel data transmission,” Bell syst. Tech. J., vol.45, pp. 1775–1796, December 1966], [Saltzberg, B. R. “Performance of an efficient parallel data transmission system”, IEEE Trans. Comm. Technol., vol. COM-15, December 1967]. Possible benefits following from multi-carrier modulation have been mentioned in many articles [Meuller, T. Brueninghaus, K. and Rohling H. “Performance of Coherent OFDM-CDMA for Broadband Mobile Communications”, Wireless Personal Communications 2, Kluwer Academic Publishers, 1996, pp. 295–305], [Kaiser, S. “OFDM-CDMA versus DS-CDMA: Performance Evaluation for Fading Channels”, ICC '95, pp. 1722–1726]. Numerous theoretical publications have been written on said attractive modulation technique [Kalet, “The multitone Channel”, IEEE Trans. Commun., vol. 37, no. 2, February 1989], [Fazel, G. Fettweis, “Multi-Carrier Spread-Spectrum”, Kluwer Academic Publ., 1997]. Specifically in multipath fading propagation situations, such as for example encountered in an indoor environment, multicarrier modulation is a beneficial technique. Indeed, thanks to the insertion of a guard interval containing a cyclic prefix, it enables a very efficient way of combatting ISI, being intersymbol interference. Moreover, adaptive loading techniques make it possible to considerably increase the throughput performances [L. Van der Perre, S. Thoen, P. Vandenameele, B. Gyselinckx, M. Engels. “Adaptive loading strategy for a high speed OFDM-based WLAN”. In Globecom '98. Sydney, Australia, November 1998]. However, for a given carrier modulation, the bandwidth efficiency in terms of bits/sec/hertz is fixed. Given the massive growth of wireless communication and the importance of broadband services, the spectrum becomes increasingly scarce. One method to increase the capacity or the bandwidth efficiency of a wireless system, is to apply cellularization in order to reuse spectrum in different non-interfering cells. While this technique has been applied successfully in mobile telephone networks, it is—from an economic point of view—inappropriate for small- or medium-scale indoor networks as WLANs or home LANs. First of all, high operating frequencies (i.e. millimeter wave band) would be required to achieve a reasonable reuse factor [M. Chiani, D. Dardari, A. Zanella, O. Andrisano. “Service Availability of Broadband Wireless Networks for Indoor Multimedia at Millimeter Waves”. In ISSSE '98. pp. 29–33, Pisa, Italy, September 1998][T. Ithara, T. Manabe, M. Fujita, T. Matsui and Y. Sugimoto. “Research Activities on Millimeter-Wave Indoor Wireless Communications”, in ICUPC '95, Tokyo, Japan, November 1995]. Secondly, cellularisation introduces an extra layer of hierarchy and complicates the protocol stack. Thirdly, cellularisation increases the installation effort. An alternative method that allows spectrum reuse and which has none of the disadvantages of cellularisation, is the application of Space Division Multiple Access (SDMA) techniques [A. Paulraj, C. Papadias. “Space-Time Processing for Wireless Communications”, IEEE Signal Processing Magazine, pp. 49–83, November 1997]. Making use of an antenna array, SDMA can separate different users communicating over the same frequency band and at the same time, by exploiting their distinct spatial signature. As such, it allows reuse within one cell of the cellularized space. SDMA has been proposed for single-carrier systems, where its benefits have been extensively proven [G. Tsoulos, M. Beach and J. MacGeehan, “Wireless personal communications for the 21st century: European technological advances in adaptive antennas”, IEEE Communications Magazine, Vol. 35, No. 9, pp. 102–9, September 1997],[R. Roy, “An overview of smart antenna technology and its application to wireless communication systems”, in IEEE International Conf. On Personal Wireless Communications, pp234–8, New York, N.Y., 1997],[S. Jeng, G. Xu, H. Lin and W. Vogel, “Experimental study of antenna arrays in indoor wireless applications”, in Asilomar Conference on Signals, Systems and Computers, pp. 766–70 Los Alamitos, Calif., 1996]. However, these single-carrier SDMA systems for high speed (e.g. 100 Mbps) wireless systems demand a massive amount of processing (e.g. in the order of Gflops) [P. Vandenameele, L. Van der Perre, B. Gyselinckx, M. Engels and H. De Man, “An SDMA Algorithm for High-Speed Wireless LAN”, in Globecom 98 Sydney, Australia, pp. 189–194, November 1998]. The combination of OFDM as a modulation technique with an antenna array is known in the art [G. Raleigh and J. Cioffi, “Spatio-Temporal Coding for Wireless Communication”, IEEE Transaction on Communications, Vol. 46, No.3, pp. 357–366, March 1998]. However, these algorithms are limited to a single user scenario and do not enable SDMA.
In “A spread-spectrum multi-carrier multiple-access system for mobile communications”, Kluwer Academic Publishers, Dordrecht, The Netherlands, Multi-Carrier Spread-Spectrum, 49–56, 1997, S. Kaiser and K. Fazel describe a point-to-point approach wherein both the transmitting and receiving peer have one antenna. Different users are handled by FDMA, each user having a disjunt set of carriers. The data of a single user id spread over a subset of carriers using spread spectrum techniques.
In WO97/416647, separation of the users is based on the spread spectrum code together with channel info. Operation of a ZF or MMSE combiner is carried out over all carriers at once.
A. Kuzminskiy et al. disclose in “Multistage semi-blind spatio-temporal processing for short burst multiuser SDMA systems”, Conference on Signals, Systems & Computers, Vol 2, 1–4 November 1998, pages 1887–1891, a successive interference cancellation technique for SDMA in the context of a mixed blind/training-based detection approach. It does not rely on subband processing such as OFDM. Further, it does not use a multicarrier approach. From a frequency point of view, it exploits a complete band. The selection of the determination of the signal of one user will affect the signal for the complete frequency band.